This invention relates to a level or power control device for use in digital modulation accompanying an amplitude variation, such as in .pi./4 quadrature phase shift keying (QPSK) modulation.
Various level control devices are already known for use more in general. Examples are described in Japanese Patent Prepublication (Kokai) No. 157,927 of 1992. According to this prepublication, an intermediate frequency (IF) signal is preliminarily mixed with a local signal into a radio frequency (RF) signal for supply to a prior-art level control device. In an improved level control device revealed in the prepublication, an IF signal is supplied directly to the level control device. The improved level control device produces an RF signal with a controlled level.
Meanwhile, conversion of analog communication to digital communication is in rapid progress recently. Digital modulation with an amplitude variation is widely applied to communication networks. For example, .pi./4 QPSK modulation and demodulation is becoming into practical use in a mobile telephone and/or other communication network.
In general, a level control device is supplied with a device input signal and produces a device output signal having a controlled or predetermined output level. For use in digital modulation accompanying an amplitude variation, a novel level or power control device is invented by the present inventor and disclosed in a patent application which is filed in Japan under Patent Application No. 4-145,302 of 1992 and which was published under Japanese Published Unexamined Application No. 5-343938. Although this patent application was filed on the same date as a basic patent application used in filing the instant patent application abroad, the novel level control device will be used as showing the state of art on describing the present invention.
In the manner which will later be described in greater detail, the novel level control device comprises a variable attenuator responsive to the device input signal for producing an attenuator output signal having an attenuated level, and a level detecting unit for detecting the attenuator output signal to produce a detected signal having a detected voltage related to the attenuated level. Supplied with the detected signal as a generator input signal and comparing a digital value of the detected voltage with a reference value, a control signal generator generates an attenuation control signal having an attenuation control level.
More in detail, the control signal generator comprises an A/D converter for converting the detected voltage to the digital value. Comparing the digital value with the reference value, a processor produces a digital control signal having a digital control value. Converting the digital control value to an analog value, a D/A converter produces the attenuation control signal with the analog value used as the attenuation control level.
Controlled by the attenuation control signal, the variable attenuator enables use of the attenuator output signal as the device output signal. The reference value is readily determined by the controlled output level. It is possible to use whichever of the IF and the RF signals as the device input signal.
When used in time division multiplex (TDM) modulation, the novel level control device is very effectively operable. It is known that the TDM modulation is desirable in effectively using channels.
The novel level control device, however, has a few defects when such a level control device should be reliably operable, namely, be operable stably and precisely or correctly. More particularly, operation of the novel level control device decomes unstable and incorrect when the controlled output level is low, typically, when the controlled output level is not higher than plus 10 dBm.
One of the defects is such that the level control is unstable against a variation in the ambient temperature. The level detecting unit includes a detector diode for detecting the controlled output level which is derived from the attenuated level. When the controlled output level is low, the detector diode has a high internal resistance and becomes excessively sensitive to the temperature variation.
In such a level detecting unit, the detector diode is usually supplied with a constant bias current with a bias voltage. When the controlled output level is low, the detected voltage is subjected to an objectionably wide variation by a slight variation in the contolled output level.
Another defect is an appreciable influence caused to the attenuation control level by conversion errors inevitable in operation of the A/D and the D/A converters. When the controlled output level is low, the conversion errors are not negligible as compared with a little variation in the detected voltage.
The variable attenuator usually comprises attenuator diodes and has an attenuation characteristic which is nonlinear relative to the attenuation control level. When the controlled output level should be low, the variable attenuator must give a great amount of attenuation to the device input signal. For this purpose, the attenuation control level must be given a small value. Consequently, it again results that the conversion errors are not negligible when compared with this small value of the attenuation control level.